Liquid Metal Processing Group
Metallurgists recognized almost century ago that the continuous casting of metal shapes offered potential economies significantly better than that possible with closed mold casting in terms of higher production rates, lower labor costs and less extensive installations per unit of invested capital.(1) However the technical difficulties involved in large-scale commercial production were not overcome until the 1950's, and only for billets and slabs. Continuous casting of billets and slabs is now so well understood that it is among the principal means of metal solidification at large integrated primary metal facilities. Nevertheless the additional and considerable savings that might be realized by minimizing subsequent reduction to plate and sheet stock are still not forthcoming.
The commercial continuous casting of plates and sheet stock still poses considerable problems, generally involving spanwise temperature gradients in the liquid metal which translates into almost imperceptible thickness variations across the sheet, but variations that can nevertheless significantly surface acceptability and complicate subsequent reduction. Those proposing continuous sheet casting processes generally did not appreciate the severity of this problem, although otherwise their proposals were quite practical.(2,3) It is only recently that these difficulties are being seriously addressed. The solutions range from baffling systems in the liquid metal feed arrangement to minimize lateral temperature gradients to drastically limiting the width of the strip produced.
The following is a concise overview of existing continuous casting processes dedicated to non-ferrous strip production. They can be conveniently divided into drum casting and belt casting categories. Unlike the vertical casting machines generally used in the steel industry, non-ferrous machines are horizontal casters. In virtually all cases the the limitation on sheet width is related to gauge uniformity concerns.
All of the drum casters limited production capability because of limited contact area between the emerging sheet and the drum for heat extraction. The single drum Honeycutt machine shown in Figure 1 employs a casting drum upon which molten metal directly freezes to strip. Although this machine would appear to be reasonably simple in concept the tundish is rather complicated.(4) The heated gauging drums are provided to insure that only a uniform film of liquid metal reaches the casting wheel, thereby providing a smooth surface applicable to further processing. Accordingly the gauging rolls do not contact the solid sheet surface.
Although this machine would appear to be reasonably simple in concept the tundish is rather complicated. The casting drums are coolest at their ends because the molten metal is constrained by the sidewalls of the tundish. Consequently the metal pool has least superheat at the lateral walls as shown in Figure 3 and therefore freezes more rapidly than the hotter metal nearer the spanwise center of the tundish. Because of the minimal superheat at the ends of the molten pool the edges of the cast strip tend to be thicker than the nominal gauge of the strip.
Fig. 1.1 Honeycutt Continuous Casting Machine
To minimize such spanwise temperature gradients in the molten metal a baffling system within the tundish is required to induce spanwise mixing of the molten metal. The baffling can be quite complex however, and several schemes have been advanced to satisfy the temperature uniformity requirements.(5,6) Flexibility is lost in that operational changes such as sheet thickness and casting rate require altering the baffling arrangement, as does different alloys. Heating the two ends of the drums by a gas flame has been proposed, which adds considerable complications to an ostensibly simple concept.(7)
The twin-drum Hunter process shown in Figure 2 is used primarily for strip casting of commercially pure aluminum for subsequent rolling to foil. Because liquid metal begins to freeze at the drums before the nib is reached, a compressive load is applied to the emerging strip. Although the result is a uniform gauge the resulting deformation can cause undesirable second-phase segregation when the twin-drum process is applied to commercial alloys, as might be required for the production of beverage can stock.
Fig 1.2 Hunter Continuous Casting Machine
All of the belt casters are comparatively complex. Of the belt casting machines the Hazelett machine shown in Figure 3 is the most widely used.(8) This machine freezes a stream of molten metal between water-cooled stainless steel belts as shown in Figure 1, accomplishing simultaneous freezing of the opposite sheet surfaces.(9) Accordingly this machine directly eliminates the major problem associated with direct casting of molten metal against a revolving drum: longitudinal and spanwise variations in sheet gauge arising from temperature gradients in the molten metal.
The Hazelett machine however is exceedingly complex. Belt tensioning and tracking require a complete hydraulic system for several sets of belt tensioning cylinders o prevent belt wandering, with tensioning and tracking requiring continual monitoring and regulating.(10)
Fig 1.3 Hazelett Continuous Casting Machine
In addition, cooling requires a manifold and spraying system to supply impinging water on the reverse sides of the belts and means to prevent cooling fluid from contacting the molten metal. Preventing any contact between the fluid flowing through the liquid-cooled belt quenching tanks and the freezing metal is imperative, and the sealing system can become quite involved.(11) Moreover, moving side dams are required to constrain the molten metal between the drums. Not surprisingly continual and often minüte adjustments are necessary for proper operation of the Hazelett machine.
The Harrington machine, unlike the Hazelett machine, does not employ direct cooling of the freezing strip through the belt as shown in Figure 1.4.(12) Instead the belt quenching box is located at a distance from the molten metal, with the machine thereby utilizing the belt as a heat sink. As expected, uniform cooling of the belt and complete removal of the cooling fluid is critical.(13)
FIG 1.4 Harrington Continuous Casting Machine
The lack of direct belt cooling permits only very thin strips to be cast so as not to overwhelm the cooling capacity of the belts. Nevertheless the belt entrance temperature can be as high as 200° C (392° F) and exit temperature almost 575° C (1070° F). However because the strip produces is hot it can be directly rolled for subsequent processing and annealing. The greatest limitations on the Harrington machine is that it is limited to producing not only very thin strip but equally limited to producing only very narrow strip.
Controlled cooling of the strip is achieved with the Gyöngyös Machine by using chilling blocks as heat sinks as shown in Figure 1.5, permitting control of the solidification rate without cooling the strip below 500° C (932° F).(14) In this manner a highly uniform and effective heat extraction is achieved. Consequently this process is presently used in commercial production of aluminum sheet. However, unless the blocks are in uniform contact with the emerging plate and each other, without gaps between the blocks into which molten metal can penetrate, flash can appear on the plate surface, resulting in surface defects after rolling. Hence a high degree of process control is necessary.
FIG 1.5 Gyöngyös Continuous Casting Machine
All continuous casting machines suffer to some degree from non-uniform metal flow through the tundish, possibly involving stagnation points which can result in temperature gradients and, if not controlled, in spanwise gauge variations. Moreover, unless the freezing strip maintains intimate contact with the casting surface during the cooling process, localized surface reheating can occur which can alter particle precipitation patterns and dentritic arm spacing, both of which can be detrimental to further processing and adversely affect surface quality. The advantages and limitations of the various continuous casting machines described are tabulated below.
I. Comparison of Continuous Casting Machines
In light of the many difficulties inherent to current direct casting machines
a propietary continuous casting apparatus is disclosed herein in which
process operation, including transverse temperature gradients, can be microprocessor
ALTERNATIVE CONTINUOUS SHEET-CASTING PROCESS
According to the proprietary Quad-Cast Continuous Casting (QCCC) process shown in Figure 2.1 a liquid metal pool is constrained between a pinched pair of internally water-cooled casting drums.(15)
Fig. 2.1 Quad-Cast Molten Metal Reservoir
The sheet is cast on the upper surface of the twin casting drums, emerging as twin streams in opposite directions. Unlike other drum casters, a significant portion of the drum surface remains in contact with molten metal. The pool is supplied from a spanwise manifold that directs the molten metal towards the pinch. Hence the solid metal begins to freeze on the drums immediately beyond the pinch and is complete at the meniscus. The dam allows only clear surface metal to reach the gauging rolls. The heated gauging rolls insure that only a uniform film of liquid metal reaches the solidifying sheet and dresses the sheet surface.
Fig. 2.2a Quad-Cast Continuous Casting Process
Because lateral liquid-metal temperature is highly regulated the QCCC process permits very wide precisely-gauged non-ferrous sheets to be continuously cast.
Fig. 2.2b Quad-Cast Continuous Casting Process
Unlike any of the casting machines previously described, the proprietary QCCC machine circulates a large volume of molten metal between the casting drums, promoting uniform deposition on the drums and alloy homogeneity. Figure 2.3 illustrates the probable circulation. Because of boundary layer drag in proximity to the drums there are no stagnation points in the molten pool. The principal determinant of sheet thickness is the heat-transfer rate during the period the metal pool is in contact with the drums: essentially the circumferential point between the pinch and the meniscus at which the sheet detaches itself from the casting drum. The principal determinant of surface quality during casting is the temperature of the drums and the adherence of the sheet to the drums, the latter promoted by hydrostatic clamping.
Fig 2.3 Presumed Fluid Circulation
It is imperative for surface quality that the solidifying sheet be maintained at a relatively uniform temperature along its length and through its thickness during casting, as in the Gyöngyös Machine. Hence the casting drums should not be subject to wide temperature excursions, particular as they enter the molten pool at the pinch. In this regard the casting drums are constructed of copper for heat conductivity with heavy walls for heat uniformity, as shown in Figure 2.4. The surface is hard-plated and fine polished for surface smoothness.
Fig 2.4 Water-Spray Cooling System
The interior surface of the casting drums opposite to the molten pool are water-spray cooled by a spray head as shown in Figure 2.4 so as to maintain the contact surface of the emerging sheet at a relatively uniform temperature. The gauging rolls maintain the temperature of the upper surface roughly equal to that of the lower surface. For this purpose the casting drums and the gauging rolls are held at roughly the same temperature. To minimizing surface porosity arising from excessive dentritic arm spacing at the sheet surface the emerging sheet is held above 500° C (932° F).
Fig 2.5 Surface Quality Control
Unlike other drum casting machines wherein drum-liquid contact is considerably less than 45° of the drum circumference, for the QCCC machine contact extends over a full 90°, allowing more uniform cooling, much like belt casting machines. As shown in Figure 2.5, the gauging rolls insure that only a uniform film of liquid metal reaches the upper sheet surface as it emerges from the pool. Consequently the rolls do not impose a compressive load on the solid sheet surface which can cause adverse precipitation at the centerline. While only a small area of the gauging rolls are in contact with the molten metal, only a small amount of metal needs to be solidified.
The dams allow only fresh surface metal to reach the gauging rolls with the surface dross siphoned off at regular intervals.
The solidifying sheet must be kept in intimate contact with the casting drums for efficient heat transfer, otherwise any localized space opening between the sheet and the drum during casting would allow local reheating of the sheet surface, possibly distorting the surface at best or causing melt-through at worst. In this regard the molten metal pool exerts a substantial hydrostatic pressure on the solidifying sheet, aiding in maintaining contact between the sheet and the drum.
However another difficulty arises concerning spanwise temperature uniformity
which is shared to some degree by all continuous casting machines. The
ends of drums and the edges of belts are invariably cooler than their spanwise
centers. The result is that sheet edges are thicker from drum casters and
belt distortion occurs with belt casters, both degrading strip quality.
The remedy in most cases is to limit the width of the casting machine and
therefore the cast strip.
With the pinched-drum casting a direct remedy to spanwise temperature gradients which virtually eliminate any practical limitations on sheet width. A large mass of liquid metal is contained between the casting drums by the end plates which have low thermal conductivity to minimize transverse heat loss. Hence only the heat loss to the cooled ends of the casting drums have to be compensated for, which can be readily accomplished with immersion heaters as as shown in Figure 3.1. The heaters would not have to account for a significant energy consumption as they function solely to compensate for heat loss. With the large mass of liquid metal present temperature variations will be minimal.
High-strength liquid-metal-resistant immersion heaters fabricated of fibre-reinforced silicon carbide are now available for non-ferrous die-casting to maintain a consistent liquid-metal feed temperature. The configuration of the die-casting immersion heaters, as shown in Figure 3.1, is directly applicable to the QCCC machine.
To provide controlled superheat the pool is divided into discrete spanwise temperature stages to provide for a gradually increasing pool temperature towards the ends of the pool. In this manner decreased casting-drum end-temperature is compensate for, with each stage provided with a thermocouple sensor for process control. The favorable fluid circulation without stagnation past the heaters as shown in Figure 2.3 insures that both heat transfer from the heaters to the liquid metal is highly efficient and that the temperatures read by the thermocouples are representative, with any variations rapidly indicated, both necessary for rapid automated temperature correction.
Fig. 3.2 Sensors for Spanwise Temperature Control
The required superheat above nominal pool temperature T1
to compensate for decreased casting-drum end-temperature will be dependent
on the casting rate which in turn will depend on sheet gauge and casting
drum speed W. With instantaneous g,
and temperature differentials DT12
and DT13 accurately
sensed the microprocessor shown schematically in Figure 3.2 regulates the
casting process, including the molten metal flow to the casting apparatus.
The controllers regulate the immersion heaters to maintain the correct
stage temperatures. Such closed-loop systems have proven highly amenable
to commercial process control.
The maximum width of the of sheet produced by the PDDC machine is limited only by liquid metal supply and distribution requirements inasmuch as the several lateral temperature control stages shown in Figure 3.2 will provide sheet gauge uniformity within required limits. Accordingly process control is essentially independent of sheet width.
For example consider strip of thickness to cast on drums of diameter D. For the alloy cast the property constant P=wak/HLd where wa is the atomic weight, k the thermal conductivity, HL the latent heat of solidification of the alloy and d is its density.
The thickness t<to of the solidifying sheet at any height h<ho between the pinch and the meniscus, where ho is the height of the pool shown in Figure 2.3, can be calculated from
(1) t = 3[P(DT/W)]1/3 [sin-1(2h/D)]2/3
where W is the drum rotational speed and DT is the temperature difference through the sheet from its freezing temperature Tf at the solid-liquid interface to the drum surface temperature Tr. Hence tµDT1/3 and the sheet thickness will be uniform across the sheet to a greater proportion than DT is uniform.
The weight of liquid metal W constrained between the pinch and the meniscus of the drums can be calculated from
(2) W = wd[hoD - ho(D2/4-ho2)1/2 - (D2/4) sin-1(2ho/D)]
where w is the width of the sheet and ho<D/2.
If the molten metal fills the pool between the casting drums shown in Figure 2.3 then ho=D/2 and consequently sin-1(2ho/D)=p/2. Under these conditions equation (2) can be simplified to
(3) t = 3[P(DT/W)]1/3 [p/2]2/3
(4) W = wd[D2(2-p/2)/4]
From this information the production rate W/w of a sheet of thickness to and width w is
(5) W/w = 2[dtoDW]
the factor of two accounts for the double output of sheet stock.
A reasonable approximation can be made as to the operating requirements of the QCCC machine, particularly in light of the experience gained by those involved with the single and double drum machines. However the QCCC machine will have twice the capacity as the other drum machines with essentially the same size installation because the pinched-drum apparatus produces two sheet streams simultaneously. For example, consider the requirements necessary to cast 60,000 kg/hr (132,000 lbs/hr) of aluminum sheet 3 m (118 in) wide and 1.5 mm (0.06 in) thick on one meter (39 in) diameter casting drums.
For 30,000 kg/hr (66,000 lb/hr) of sheet per stream the required strip output speed is roughly 70 cm/s (135 ft/min), necessitating a drum speed of 13 RPM.
Equation (4) indicates that a substantial weight of molten metal is contained within the liquid metal pool. Accordingly, during casting some 900 kg (1980 lbs) of liquid aluminum, almost a tonne, is held in the liquid pool between the drums as indicated in Figure 11, and is nominally replaced in just under one minute. This significant mass of circulating metal promotes alloy uniformity during casting.
Because of the significant mass of aluminum in the molten pool and its rapid turnover a minimal transverse temperature gradient exists within the pool that necessitates compensation by the immersion heaters. Nevertheless, for a highly conservative estimation of the power required by the immersion heaters (considering the mass of liquid metal contained) it will be assumed that as much as 5% of the metal in the molten pool, roughly 45 kg (99 lbs), is cooled by as much as 10° C (18° F) by the cooler ends of the casting drums. Accordingly some 3,000 kg/hr (6,600 lb/hr) of molten metal will be affected. To compensate for this heat loss the immersion heaters would be required to supply less than 6 kW, or 3 kW to each end of the molten pool to maintain uniform the spanwise temperature stages.
The energy requirements of the immersion heaters is so small that it could be met most efficiently by electric immersion heaters rather than gas-fired tube heaters, an alternative which would require less than 25,000 btu/hr, significantly less than a typical industrial burner can supply and control. Electric heaters could be more readily controlled and require far less equipment and no plumbing, and can be designed with a resistance gradient for a longitudinal temperature variation. Because of the very favorable fluid flow in the vicinity of the heaters: downwards over the heaters to supply the metal freezing at the pinch; high heat-transfer efficiencies would be exhibited, promoting sheet thickness uniformity.
The significant mass of aluminum in the molten pool exerts hydrostatic pressure against the solidifying sheet, clamping the sheet to the casting drums. For a one meter (39 in) drum the maximum pool height is 50 cm (19.5 in), resulting in a clamping pressure at the pinch of roughly 1400 kg/m2 (2 psi). Just 10 cm (4 in) from the pool surface the clamping pressure is still 300 kg/m2 (0.4 psi). Even at this shallow depth, on a full spanwise strip one cm (0.4 in) wide a three kg (6.6 lbs) clamping load is imposed in the solidifying strip, insuring intimate contact between the solidifying sheets and the casting drums, thereby promoting fine surface quality.
There are many practical liquid metal delivery systems for the QCCC machine because spanwise temperature gradients are controlled using temperature regulation by sensors and transducers under CPU control rather than structural solutions involving installation geometry.
The strength and weaknesses of the various drum and belt casters in operation or development have been discussed in detail. To varying degrees all are intrinsically restrictive as far as sheet width is concerned because of adverse spanwise temperature gradients. The great simplicity of the drum casters is offset by their low casting rates arising from limited heat extracting capability. Although the belt casters offer much higher heat extraction capability than drum casters they are comparatively more complex, requiring precision belt tensioning and tracking means and leak-prone belt cooling boxes. In contrast the proprietary QCCC casting machine offers virtually unrestricted cast sheet width, greater heat extraction capability than other drum casters and greater simplicity than belt casters, with significantly greater production capability than either. With its heavy-walled drums, the QCCC process will have the highly uniform and effective heat extraction capability of the Gyöngyös Machine.
The proprietary QCCC machine disclosed herein has several decided advantages compared to conventional strip casting for sheet stock, including: